|The PC133 spec itself is 100% backwards compatible with PC100 and PC66 specs.|
However that's not the only thing that determines whether it will work in your mboard or be recognized as it's full size.
See response 5 in this for some info about ram compatibilty:
E.g. I have an Epox MVP3-G5 mboard that worked fine from the beginning of 2000 until the end of 2010 when two capacitors failed on it. It was designed to be used with PC100 ram but the FSB speed can be slightly overclocked, so I tried PC133 ram with it. Some 256 mb modules that had 8 chips on one side were recognized as half that size, none were recognized as 256 mb, some were not recognized at all; 256 mb modules with 16 chips - 8 chips on each side - were recognized as 256 mb.